1. Field of the Invention
The present invention relates to an optical information recording medium and a method of optically recording/reproducing optical information and. More particularly, the present invention relates to an optical information recording medium and a method of optically recording/reproducing information in which data recording density can be substantially increased.
2. Brief Description of the Prior Art
When an information signal is conventionally recorded on an optical recording medium (such as an optical disk or card), a light beam is modulated in accordance with the information signal, and the rotating optical disk (or reciprocating optical card) is irradiated with the modulated beam to form spiral, concentric or linear information tracks consisting of information pits corresponding to the information signal.
Various forms of information pits constituting the information track are known. Recording films of additional write type optical disks utilize information apertures or pits formed; in a metal- or organic-based material, by a change between crystalline and noncrystalline phases, by a change in reflectance upon formation of an alloy, and utilizing a geometric change such as a surface projection or a change in surface state.
Of these forms of information pits, a recessed pit having a depth of .lambda./4, i.e., 1/4 of a wavelength .lambda. of a light beam radiated on an optical recording medium is formed, and information signals are recorded as pits having different lengths. In the reproduction mode, when such a pit is irradiated with a light beam, a beam spot 2 is formed on the optical recording medium, as shown in FIG. 11. Since the size of the beam spot 2 is larger than the width of a pit 1, a beam reflected by the information pit 1 interferes with a beam reflected by a portion outside the information pit 1. When a portion between the information pits 1 is irradiated with the beam spot 2, it is totally reflected. When a light-receiving element of an optical head receives a beam reflected by the optical recording medium, the amount of totally reflected beam which is received is different than the amount of interfering light which is received, thus detecting the presence/absence of the information pit and thereby, an information pit array.
The optical recording medium such as an optical disk or card has many advantages, for instance, in nonvolatile properties, high recording density, noncontact medium access, flexibility, and high-speed access. However, in the technical field concerning image data and especially dynamic images, a strong demand has arisen for additionally increased recording density.
The recording density of any optical recording medium is determined by the size of a light beam spot. For example, when signals are recorded at a track pitch of 1.6 .mu.m and a bit pitch of 1 .mu.m, high density recording at a density of 6.25.times.10.sup.5 bits/mm.sup.2 can be achieved in a conventional optical disk ("Optical Disk", Mori & Kubo, P. 23, the Institute of Electronic Information). This is, however, the upper limit of recording density with these parameters.
As a method of further increasing the recording density, decreases in width of the information track and pitch can both be taken into consideration. However, this requires high-precision techniques for controlling the pitch constant and also, a compatible optical recording/reproducing apparatus.
When information pits constitute an array in a conventional optical recording medium, only one-channel signals are recorded in each information track and so, during reproduction, only one-channel signals are reproduced from each information track. In other words, when optical recording/reproduction is performed using an optical recording medium whose information pits constitute an array, only an information transfer rate can be used.